Electrostatic fibrous filter web

ABSTRACT

There is provided a nonwoven electret fiber filter formed with a nonwoven web of electret fibers needle-punched to an open scrim support. The scrim support material has individual discrete open areas with an average cross-sectional area as viewed from the plane of the filter media of at least 0.25 mm 2 , preferably 1.0 mm 2 , and a pressure drop across the scrim support, without the filter web, of less than 1.5 mm H 2  O measured at 98.4 meters/min gaseous face velocity. The resulting filter has enhanced lifetimes, a low pressure drop and high filtration efficiencies.

BACKGROUND AND FIELD OF THE INVENTION

The present invention relates to a process for forming an electretnonwoven filter and products of such a process.

Nonwoven webs of electret fibers are typically formed of looselyassociated electret-charged fibers. The filters can be electrostaticallycharged prior to, during, or after being formed into a nonwoven web. Forexample, post-formation charging is described in U.S. Pat. No. 4,588,537which charges a lofty nonwoven web which can be formed by a variety ofmethods including carding and melt blowing. The webs are charged whileunder compression and then permitted to return to their original loft.

Fibers can also be charged while they are being formed as disclosed inU.S. Pat. No. 4,215,682 (Kubik et al.), where melt-blown fibers arebombarded by ions or electrons immediately after being extruded frommelt-blowing orifices. The fibers solidify extremely rapidly in theatmosphere and are collected as a semi-coherent mass of entangledmicrofibers as the fiber web. The fiber webs are described as preferablyopen to provide a low pressure drop for liquid passing through a filterformed of the fibrous web.

Fibers can also be charged as described in U.S. Pat. No. 4,798,850. Thispatent describes blending different fibers together, which when properlyselected will induce an electrostatic charge in the fibers as thefibrous web is formed. Other patents relating to charging fibers orfibrous webs include U.S. Pat. Nos. 4,904,174; 5,122,048; 5,401,446; and4,592,815.

A particularly effective method of forming a nonwoven electret fiberfilter web is described in U.S. Reissue Pat. No. 30,782 (Van Turnout etal.). The electret fibers in this patent are formed from a coronacharged film that is fibrillated to form the charged fibers. The chargedfibers can then be formed into a nonwoven filter web by common methodssuch as carding or air laying. This charging method provides aparticularly high density of injected charges in the finished fibers.However, problems are often encountered with forming webs from theseprecharged fibers. The fibers are generally quite large and uncrimped.They also have a resistance to bending. Due in part to these properties,the fibers resist formation into a uniform coherent web, particularly atlow basis weights. This problem is partially addressed in U.S. Pat. No.4,363,682, which proposes the use of such fibrillated fiber webs in facemask applications. In order to provide a more coherent web, as well asone that resists shedding fibers, this patent proposes a post-embossingtreatment. This post-embossing welds the outer surface fibers togetherallegedly providing a more coherent and comfortable web for use as aface mask. However, this treatment will also tend to result in a morecondensed web, which would increase pressure-loss across the filter weband decreases filter life.

An improvement over the embossing treatment is disclosed in U.S. Pat.No. 5,230,800. This patent proposes needle-punching the fibrillatedelectret fiber filter webs (e.g., prepared via the Van Turnout et al.method) onto a scrim support. The result is a consolidated coherentfibrous filter composite material with improved uniformity andfiltration performance.

The present investigator was concerned with providing a nonwovenelectret fiber filter with long filter lifetimes and low pressure dropswithout loss of filter performance, which filter material can beeconomically and simply manufactured and easily converted into the finalfilter form.

BRIEF DESCRIPTION OF THE INVENTION

A nonwoven electret fiber filter media and filter is obtained by formingthe filter media using a nonwoven web of electret fibers on an openscrim support. An unsupported nonwoven fibrous filter web is placed ontothe open, substantially non-extensible, scrim support material. Thescrim support material has individual discrete open areas with anaverage cross-sectional area as viewed from the plane of the filtermedia of at least 0.25 mm², preferably 1.0 mm², and a pressure dropacross the scrim support, without the filter web, of less than 1.5 mm H₂O measured at 98.4 meters/min face velocity. The unsupported fibrousfilter web and the scrim support are joined to form the filter media byneedle-punching the filter web and scrim support material to provide ahighly uniform nonwoven fibrous filter media with enhanced lifetime andfiltration performance. At least certain of the fibers forming thenonwoven fibrous filter web are provided with an electret charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred process arrangementaccording to the invention method.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to an improvement over the inventiondisclosed in U.S. Pat. No. 5,230,800, which describes a method ofcombining a fibrous web of fibrillated electret fibers and a supportscrim by a needle-punching operation in a manner in which produces afilter media which is extremely uniform in its physical and performancecharacteristics. However, the present inventor was concerned not only inproviding a filter media which had uniformity of properties as discussedin the above U.S. Pat. No. 5,230,800 but also one which would provide ahigh level of filtration efficiency at a relatively high gaseous facevelocity over an extended lifetime. Particularly, of concern wasproviding a superior filter media for use in forming a high performancefurnace filter, or a like general purpose air-cleaning filter which mustefficiently filter large volumes of air for as long a lifetime aspractically possible. Preferably, the filter media should also performwell at high gaseous face velocities (e.g., greater than 250 m/min) suchthat it can be used either as a flat filter, or a pleated filter forenhanced performance. Unexpectedly it was found that by proper selectionof the scrim support to which a fibrous filter web layer is joined byneedle-punching a nonwoven fibrous filter media with electret fiberscould be formed which had both high filtration performance at highgaseous face velocities and an extended lifetime.

Specifically, the invention filter web scrim support should be anextremely open material having a large number of discrete open areas,which open areas pass through the scrim from one face to the oppositeface. These discrete open areas should have an average cross-sectionalarea of at least 0.25 mm², most preferably at least 1.0 mm², however,the individual open areas can range in size from 0.1 mm² up to 10 mm² orlarger. Preferably, the open areas have a non-tortuous path through thescrim, most preferably the open areas extend directly from one face tothe opposite face(e.g., as a column). Generally, the ratio of open areaaverage pathlength through the scrim to the average scrim thickness isfrom 3 to 1, preferably from 2 to 1 and less. The scrim open area canalso be described in terms of an Effective Circular Diameter (ECD) whichis the diameter of the largest circle that can fit into an individualdiscrete open area. The average ECD is generally at least 300 μm,preferably at least 500 μm. Despite the extremely open nature of thescrim support material, it should be reasonably strong, generally havinga tensile strength of at least 50 kg/m, preferably at least 100 kg/m.The overall pressure drop of the scrim material should be relativelysmall in comparison to the pressure drop across the electret-chargedfilter web material (e.g., less than 50 percent preferably less than 30percent of the filter web pressure drop) and generally will have apressure-drop of less than 1.5 mm H₂ O, preferably less than 1.0 mm H₂O, and most preferably less than 0.5 mm H₂ O at a gaseous face velocityof 98.4 meters/min at ambient conditions.

The scrim material can be formed of any suitable material such as athermoplastic polymer, ductile metal or the like. Preferably, the scrimis formed of thermoplastic fibers such as a scrim or netting materialsuch as the cross-laminated polyethylene fibers sold under the tradename CLAF by Amoco. Other cross laminated fiberous webs could also beused, with the lamination done by conventional techniques such as heat,sonics or adhesive lamination. If the final filter is to be pleated, thescrim is also preferably pleatable so that additional pleating layersneed not be used, however, conventional pleatable layers can be used ifdesired.

The fibrous filter web layer is a nonwoven fibrous web where at least aportion of the fibers forming the web are individually provided with anelectrostatic charge, generally referred to as electret fibers. Theseelectret fibers can be charged by known methods, e.g., by use of coronadischarge electrodes or high-intensity electric fields or bytribo-charging (as described in U.S. Pat. No. 4,798,850). The fibers canbe charged during fiber formation, prior to or while forming the fibersinto the filter web or subsequent to forming the filter web. The fibersforming the filter web can even be charged subsequent to being joined tothe scrim support layer. The nonwoven fibrous filter web can be formedby conventional nonwoven techniques including melt blowing, carding,Rando, spin bonding or the like, preferably the web is not consolidated(e.g., by hydroentanglement, heat or sonic bonding or the like) but canbe if desired.

The fibrous filter web layer or layers can have a total basis weightranging from 10 to 400 grams/M², preferably from 20 to 150 grams/M² forfurnace filter applications. Generally, the filter layer or layers havea combined total pressure-drop of less than about 10 mm H₂ O, preferablyless than 6 mm H₂ O, at a gaseous face velocity of 98.4 meters/min. Thepressure drop of the filter layer will generally be at least 1 mm H₂ O.

FIG. 1 represents an apparatus arrangement 1 for practicing the presentinvention method of manufacture. A nonwoven filter web layer 10 issupplied from a roll 2 which could also be the web former (e.g., acarding machine or other web forming devices). Additionally, a secondfilter web layer could be taken off a second roll and joined to web 10.This allows for greater flexibility in the choice of basis weights. Thescrim support 11 is fed from a supply roll 4 onto the fibrous filter weblayer 10. However, the scrim support 11 could also be fed upstream ofthe supply roll 2 so that the fibrous filter web layer 10 is laid ontothe scrim support 11 or between two scrim support layers 11. A secondsupply roll 2' can be used so that the scrim support 11 is a centerlayer between two outer nonwoven filter layers. This is shown in FIG. 1by a second supply roll with substantially identical numbering (10' and2'). Likewise, a second fibrous filter web layer could be taken off anadjacent supply roll and joined to web 10' to allow for adjustment inthe basis weight. The two fibrous filter web layers 10 and 10' can be ofdiffering basis weights.

The fibrous filter web layer 10, or layers, and scrim support 11 arethen fed to a needling station 5 where the filter web layer 10, orlayers, is joined to the scrim support 11 by the action of the needlesto form the filter media 12. The needles will preferably penetrate a topfilter web layer before a scrim support layer to transversely displacefibers securely down into a scrim support, and possibly up into anoverlying scrim support, and promote the fiber interlocking with thescrim and with fibers of an underlying filter web layer 10'. The needlescan be arranged to penetrate the filter media 12 composite from betweenabout 10 to 300 penetrations per cm². Higher needling densities tend tocompact the filter media 12, increasing pressure loss through the filtermedia 12. Preferably, the needling is less than 75 penetrations per cm².The needle-punched joined composite filter media 12 is then collected ontake-up roll 6 for subsequent converting into individual filter units byconventional techniques.

The filter webs can be charged in a preferred embodiment by the methoddescribed in U.S. Pat. Reissue Nos. 30,782 and 31,285. The electretfibers forming the filter web are formed from an electrostaticallycharged film (e.g., by a corona discharge electrode) that has beenfibrillated to provide electret fibers which have a substantiallyrectangular cross-section. However, the fibers forming the filter webcan be charged by any known charging method such as those described inthe Background of the Invention section above.

The electret fibers are preferably formed from a dielectricthermoplastic polymer that is capable of being charged. Suitablematerials include polyolefins, such as polypropylene, linear low densitypolyethylene, poly-1-butene, polytetrafluoroethylene,polytrifluorochloroethylene, poly(4-methyl-1-pentene) orpolyvinylchloride; aromatic polyarenes such as polystyrene;polycarbonates; polyesters; and copolymers and blends thereof. Preferredare polyolefins free of branched alkyl radicals and copolymers thereof.Particularly, preferred are polypropylene and polypropylene copolymers.Various functional additives known in the art can be blended with thedielectric polymers or copolymers such as poly(4-methyl-1-pentene) astaught in U.S. Pat. No. 4,874,399, a fatty acid metal salt, as disclosedin U.S. Pat. No. 4,789,504 or particulates, as per U.S. Pat. No.4,456,648 as well as conventional stabilizers (e.g., heat or U.V.stabilizers), fillers, cross-linking agents or the like as long as theyhave minimal adverse impact on the electret charging capacity of thepolymer in film or fiber forms.

Additional porous layers, such as woven or nonwoven layers, can beattached to the filter such as reinforcing support scrims (e.g., aspunbond layer), prefilter layers, pleatable layers, cover webs and thelike. However, these additional layers should be sufficiently open so asnot to adversely effect the overall pressure-drop of the filter. Theseadditional layers can be laminated to the filter by conventional meanssuch as by adhesives, point bonding or the like.

The invention filter media can be used in any conventional air filterbut is particularly useful in air filters used to filter large volumesof air such as furnace filters, automotive cabin filters and room aircleaner filters. The invention filter media can be incorporated into thefilters in any conventional manner in either a flat form or pleated.

EXAMPLES

Physical properties of various support scrims used in the examples aredetailed in Table I and porosity characteristics are detailed in TableII. In these Tables, Sample A is CLAF (a cross laminated web ofpolyethylene fibers) HS-9107, B is CLAF 2S-1501, C is CLAF HS-1701 and Dis LUTRASIL 10 gm/m². The LUTRASIL (a nonwoven spun bond fiber scrimmaterial) scrim is available from Firma Karl Freudenberg, Kaiserbautern,Germany. The CLAF scrims are available from Amoco/Nisseke and all weremetalized with aluminum. The CLAF scrims have lower initial pressuredrops than the LUTRASIL scrim even with significantly higher basisweight CLAF scrims.

For Table I, the basis weights given are the average of five samples.The scrim thickness was determined by combining five samples, measuringthe thickness using a "Parallel Plate Thickness Tester" using 0.1grams/cm pressure and by dividing the measured value by 5. The pressuredrop is determined by combining five samples, measuring the combinedsamples on the AFT Model 8110 (available from TSI, Inc., Minneapolis,Minn.) automated filter testing machine through a sample area of 9.65cm² at a volumetric flow rate of 100 liters/min to yield a 98.4meters/min face velocity. Tensile strengths are measured on an Instronmodel 1122 using a sample size of 2.54 cm by 17.8 cm at a crossheadspeed of 288 cm/min.

The scrims (A through C) webs were then analyzed using a Leica QuantametQ-570 Image Analyzer with a macro lens. Scrim D was analyzed using theQ-370 and a video microscope and incident light (illuminated from anangle). For the scrims samples (A through C) transmitted light was usedto highlight the open areas. A sample area of 198.6 mm² was scanned onthe CLAF scrim materials while an area of 29.24 mm² was viewed on theLUTRASIL material. Four areas on each sample were scanned or viewed andthree samples of each support scrim where tested. The Q-570 camera onlymeasures those open areas that are in focus and therefore are in thesame measurement plane. For each sample (A through D), the Q-570directly measured the number of individual open areas or openings, thelength, breath and perimeter of the openings and the area of theopenings were measured. From the measured values the average area of anopening, average aspect ratio and the Effective Circular Diameter werecalculated. Table II reports for each scrim A through E the averagenumber of openings and the average area (calculated value) of theindividual openings as well as the average length, average breath,average perimeter, average aspect ratio (calculated value) and averageEffective Circular Diameter (ECD) (calculated value).

The CLAF scrims A through C and E (E is a non-metalized version of C)have a porosity characteristic that provides for up to 300 times moreopen area than the LUTRASIL scrim (D). Also, the Effective CylinderDiameter, which is calculated from the area characteristics of each openarea, also shows that the CLAF scrims have from 14 to 19 times largerEffective Cylinder Diameter than the LUTRASIL scrim.

Example 1

A scrim supported nonwoven electret-charged fibrous filter media, wasmade according to the process described in U.S. Pat. No. 5,230,800(Counterexample 1D). It is made using the 10 gm/m² spunbond scrimLUTRASILT (scrim D) (available from Karl Freudenberg, Kaiserlautern,Germany). The filter web used in the filter media containedapproximately about 35 gm/M² of fibrillated film electret fibers madeaccording to U.S. Reissue Pat. Nos. 30,782 and 31,285. Further, threeidentical filters were made except that they used the CLAF scrims (madeby Amoco/Nisseke and metalized with aluminum) (scrims A through C andexamples 1A through 1C). These scrims were needled with a nominal 35gm/m² of the above fibrillated film electret fiber filter webs asdescribed for the filter media prepared using the LUTRASIL scrim above.Filter media was taken from all samples lots for analysis andcomparison. The results are set forth in Table III.

The total basis weight of each example filter media was determined byweighing a disc with an area of 100 cm² and converted to gm/m². Theaverage basis weight of the filter web portion of the samples wascalculated by subtracting the scrim weight from the total measured basisweight of the filter media. The pressure drop and penetration aremeasured on the AFT Model 8110 tester, based on a test area with adiameter of 3.8 cm, measured at a test velocity of 100 liters/min (thisproduced a face velocity equivalent to about 98.4 m/min or 300 ft/min).The challenge air contains an NaCl aerosol. The challenge concentrationwas calibrated and measured to be 14.66 mg/m³. The "percent penetration"is the ratio of NaCl particle concentration measured downstream andupstream of the filter multiplied by 100. Each test, or test cycle,yields a "pressure drop" and "initial percent penetration" value. TheAFT Model 8110 is then cycled continuously (challenged with the sameconcentration of NaCl particles) until a specific pressure drop (12.5 mmH₂ O) is reached. The "Number of Cycles" it takes to reach this specificpressure-drop is considered to be a direct indication of the filterlife. This pressure drop is selected as the end of the filter life(i.e., the "Number of Cycles"), since most furnace filter manufacturersconsider a filter to need changing when the pressure drop reaches 12.5mm H₂ O at a face velocity of 59 m/min through a pleated filter, or 98.4m/min through a flat media.

The Quality Factor Q is defined mathematically by the expression:##EQU1## where %P is the percent penetration, ΔP is the pressure drop inmm H₂ O, and ln indicates the natural logarithm. This value is alwayspositive and increases with reduced penetration. Conversely, as pressuredrop increases Q is reduced. Q is generally an index which isindependent of the basis weight. Thus, Q may be used to compare thefiltration performance of webs of different basis weights. All theresults are the average of three tested samples except for Example 1Cwhich was the average of two samples.

Table III shows that the CLAF scrim backed filters have a significantlylower initial pressure drop with comparable initial percent penetrationas compared to the control filter (sample D). However, the CLAF backedfilters have up to twice the filter life as calculated based on thenumber of cycles to an end of life pressure drop of 12.5 mm/H₂ O. TheCLAF backed filter webs also had higher Q value.

                  TABLE I                                                         ______________________________________                                               Scrim     Scrim    Scrim:                                                     Basis Weight                                                                            Thickness                                                                              Pressure Drop                                                                          Tensile Strength                           Scrim  (g/m.sup.2)                                                                             (mm)     (mm H.sub.2 O)                                                                         (kg/m)                                     ______________________________________                                        A      35.0      0.21     .931     297.8                                      B      23.1      0.15     .507     196.2                                      C      15.9      0.12     .501     123.9                                      Counter-                                                                             10.0      0.08     1.008    22.4                                       example D                                                                     ______________________________________                                    

                  TABLE II                                                        ______________________________________                                             Open                  Peri-                                                   Area    Length  Breath                                                                              meter Aspect                                                                              ECD  Openings                          Scrim                                                                              (mm.sup.2)                                                                            (mm)    (mm)  (mm)  Ratio (mm) Number                            ______________________________________                                        A    1.4     1.8     0.98  4.8   2.2   1.2  629                               B    2.0     2.1     1.2   5.8   2.2   1.5  511                               C    2.0     2.1     1.1   5.8   2.2   1.5  517                               D    0.008   0.26    0.034 0.22  2.3   0.085                                                                              785                               E    2.4     2.3     1.3   6.3   2.2   4.6  405                               ______________________________________                                    

                  TABLE III                                                       ______________________________________                                                        Basis          Initial                                               Cycles   Weight         Pressure                                                                              Initial                                       to 12.5  Filter Web                                                                             Quality                                                                             Drop    Penetration                            Examples                                                                             mm H.sub.2 O                                                                           (g/m.sup.2)                                                                            Factor                                                                              (mm H.sub.2 O)                                                                        (%)                                    ______________________________________                                        1A     23.3     30.92    0.076 4.2     73.1                                   1B     27.0     31.96    0.083 3.6     74.7                                   1C     30       31.3     0.081 3.5     76.6                                   Counter                                                                              15.7     31.90    0.060 4.8     75.2                                   example                                                                       1D                                                                            ______________________________________                                    

Example II

Filter media was made using a similar process, and tested, as describedin Example 1 using scrims A through D and 40 gm/m² of the same type offibrillated film electret charged fiber filter web.

The filter media was tested as in Example I above with the resultspresented in Table IV where each reported result is the average of fivetest samples.

Table IV again shows that filter life can be substantially increasedwhile pressure drop is decreased with a higher Q value when usingsupport scrim with large individual open areas.

                  TABLE IV                                                        ______________________________________                                                                       Initial                                               Cycles   Basis          Pressure                                                                              Initial                                       to 12.5  Weight   Quality                                                                             Drop    Penetration                            Examples                                                                             mm H.sub.2 O                                                                           (g/m.sup.2)                                                                            Factor                                                                              (mm H.sub.2 O)                                                                        (%)                                    ______________________________________                                        2A     14.4     40.17    0.063 5.9     68.8                                   2B     17.8     40.10    0.076 4.9     69.0                                   2C     20.5     40.16    0.078 4.6     69.7                                   Counter                                                                              11.8     39.83    0.072 6.4     63.2                                   example                                                                       2D                                                                            ______________________________________                                    

Example III

A tribo-charged electret filter web was made following the proceduresdescribed in U.S. Pat. No. 4,798,850. This web was made using thefollowing fibers:

70% by weight 4 denier polypropylene; and

30% by weight 3 denier polyester, T-183 from Hoechst CelaneseCorporation.

This filter web was randomized, carded and needle-tacked to a scrim,according to the procedures detailed in U.S. Pat. No. 5,230,800, per thefollowing descriptions:

Scrim D with a 27.73 gm/m² tribo-charged electret fiber web(Counterexample 3D);

CLAF 2S-1601, 18 gm/m² scrim, with a 28.75 gm/m² tribo-charged electretfiber web (Example 3A); and

CLAF 2S-1601, metalized 18 gm/m² scrim with 5 gm/m² aluminum, with a29.64 gm/m² tribo-charged electret fiber web (Example 3B).

All the formed filter media was tested for filtration properties per thetest detailed in Example I. Table V details the results from thisexample. The example filters (3A and 3B) had lower initial pressuredrops, longer life cycles to an end of life pressure drop of 12.5 mm H₂O and higher overall Q values than that of the counterexample filter(3D) which used the lower basis weight but lower average opening areaLUTRASIL scrim. The pressure drop improvement and life improvement arehigher than those experienced in the Example I filter webs which islikely due to the support scrim basis weight and pressure drop being ahigher percentage of the overall filter basis weight and pressure drop.

The various modifications and alterations of this invention will beapparent to those skilled in the art without departing from the scopeand spirit of this invention, and this invention should not berestricted.

                  TABLE V                                                         ______________________________________                                                                            Initial                                                   Fiber    Scrim Qua- Pressure                                                                             Initial                                   Cycles   Basis    Thick-                                                                              lity Drop   Pene-                                     to 12.5  Weight   ness  Fac- (mm    tration                            Example                                                                              mm H.sub.2 O                                                                           (g/m.sup.2)                                                                            (mm)  tor  H.sub.2 O)                                                                           (%)                                ______________________________________                                        3A     58.7     28.75    0.12  0.028                                                                              2.6    92.9                               3B     50.7     29.64    0.12  0.028                                                                              3.1    91.5                               Counter-                                                                             23.3     27.73    0.06  0.026                                                                              4.0    90.2                               example                                                                       3D                                                                            ______________________________________                                    

I claim:
 1. An electret fiber nonwoven filter media comprising at leastone nonwoven filter web comprising entangled fibers, at least some ofwhich are electrostatically charged electret fibers, wherein the web isjoined to at least one reinforcement scrim by needle-punching, saidreinforcement scrim having discrete open areas where the average openarea has a cross-sectional area of at least 0.25 mm² and thereinforcement scrim has an overall pressure drop of less than 2.0 mm H₂O at 98.4 meters/min, wherein said open areas extend from one face tothe opposite face of the scrim in a non-tortuous path.
 2. The electretfiber nonwoven filter media of claim 1 wherein the reinforcement scrimdiscrete open areas average cross-sectional area is at least 0.5 mm². 3.The electret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim discrete open areas average cross-sectional area isat least 1.0 mm².
 4. The electret fiber nonwoven filter media of claim 3wherein the reinforcement scrim has a tensile strength of at least 50kg/m.
 5. The electret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim discrete open areas have an average pathlength suchthat the ratio of the average pathlength to average scrim thickness isfrom 3 to
 1. 6. The electret fiber nonwoven filter media of claim 1wherein the reinforcement scrim discrete open areas have an averageaspect ratio of from 0.5 to 5 and the reinforcement scrim discrete openareas have an average pathlength such that the ratio of the averagepathlength to average scrim thickness is from 2 to
 1. 7. The electretfiber nonwoven filter media of claim 1 wherein the reinforcement scrimdiscrete open areas have an average ECD of at least 300 μm.
 8. Theelectret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim has a tensile strength of at least 50 kg/m.
 9. Theelectret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim has a tensile strength of at least 100 kg/m.
 10. Theelectret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim pressure drop is less than 1.5 mm H₂ O.
 11. Theelectret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim pressure drop is less than 1.0 mm H₂ O.
 12. Theelectret fiber nonwoven filter media of claim 1 wherein thereinforcement scrim pressure drop is less than 50 percent of thepressure drop of the filter web.
 13. The electret fiber nonwoven filtermedia of claim 1 wherein the reinforcement scrim is formed ofthermoplastic polymer fibers.
 14. The electret nonwoven filter media ofclaim 1 wherein the electret fibers comprise a fibrillatedelectrostatically charged film of a film forming polymer.
 15. Theelectret nonwoven filter media of claim 1 wherein the electret fibersare polypropylene polymers or copolymers.
 16. The electret nonwovenfilter media of claim 1 wherein the reinforcement scrim is a centerlayer between two outer layers of nonwoven filter web.
 17. The electretnonwoven filter media of claim 1 wherein the at least one nonwovenfilter web is a carded web.
 18. The electret nonwoven filter media ofclaim 1 wherein the at least one nonwoven filter web is a melt blownweb.
 19. The electret nonwoven filter media of claim 1 wherein the atleast one nonwoven filter web has a total basis weight of less than 120gm/m².
 20. The electret nonwoven filter media of claim 1 furthercomprising an outer reinforcing layer laminated to an outer face of thefilter.